Optoelectronic fiber codrawn from conducting, semiconducting, and isulating materials

ABSTRACT

The invention provides techniques for drawing fibers that include conducting, semiconducting, and insulating materials in intimate contact and prescribed geometries. The resulting fiber exhibits engineered electrical and optical functionalities along extended fiber lengths. The invention provides corresponding processes for producing such fibers, including assembling a fiber preform of a plurality of distinct materials, e.g., of conducting, semiconducting, and insulating materials, and drawing the preform into a fiber.

This application is a divisional of prior U.S. application Ser. No.10/890,948, which claims the benefit of U.S. Provisional Application No.60/487,125, filed Jul. 14, 2003, and U.S. Provisional Application No.60/539,470, filed Jan. 27, 2004, the entirety of all three of which arehereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.DAAD19-01-0647, awarded by DARPA, under Contract No. ECS-0123460,awarded by the NSF, under Contract No. DMR 02-13282 awarded by the NSF,under contract N00014-02-1-0717 awarded by ONR, under Contract No.Y77011 awarded by AFOSR, and under Contract No. DE-FG02-99ER45778awarded by DOE. The Government has certain rights in the invention.

BACKGROUND OF INVENTION

This invention relates generally to optical fibers, optical devices,electronic devices and optoelectronic devices and in particular relatesto fiber materials selection, fiber structure design, and fiber drawingtechniques for producing a fiber with desired functionality.

A combination of conducting, semiconducting, and insulating materials inwell-defined geometries, prescribed micro- and nano-scale dimensions,and with intimate interfaces is essential for the realization ofvirtually all modern electronic and optoelectronic devices.Historically, such devices are fabricated using a variety of elaboratemicrofabrication technologies that employ wafer-based processing. Themany wafer-based processing techniques currently available enable thecombination of certain conducting, semiconducting, and insulatingmaterials in small feature sizes and high device packing densities. Butin general, microfabrication techniques are restricted to planargeometries and planar conformality and limited device extent and/ormaterials coverage area. Microfabricated devices and systems also ingeneral require packaging and typically necessitate very large capitalexpenditures.

Conversely, modern preform-based optical fiber production techniques canyield extended lengths of material and enable well-controlled geometriesand transport characteristics over such extended lengths. In furthercontrast to wafer-based processing, fiber preform drawing techniques arein general less costly and less complicated. But in general,preform-based optical fiber production has been restricted to largefiber feature dimensions and a relatively small class of dielectricmaterials developed primarily for enabling optical transmission. A widerange of applications therefore remain to be addressed due to thelimitations of both conventional fiber preform-based drawingtechnologies and conventional microfabrication technologies.

SUMMARY OF THE INVENTION

The invention provides techniques for drawing fibers that includeconducting, semiconducting, and insulating materials in intimate contactand prescribed geometries. The resulting fiber exhibits engineeredelectrical and optical functionalities along extended fiber lengths. Theinvention provides corresponding processes for producing such fibers,including assembling a fiber preform of a plurality of distinctmaterials, e.g., of conducting, semiconducting, and insulatingmaterials, and drawing the preform into a fiber. The ability of theprocesses of the invention to intimately interface materials with widelydisparate electrical conductivities and refractive indices, achievesubmicron feature sizes, and realize arbitrary lateral geometries overextended lengths enables the delivery of electronic, photonic andoptoelectronic device functionalities at optical fiber length scales andcost.

Other features and advantages of the invention will be apparent from thefollowing description and accompanying figures, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are flow charts of processing steps of the fiber preformassembly and draw processes provided by the invention;

FIG. 2 is a schematic representation of a fiber preform and thecorresponding fiber produced by drawing the preform in accordance withthe invention;

FIGS. 3A-3G are schematic cross-sectional views of example fibergeometries enabled by the processes of the invention;

FIG. 4 is a schematic cross-sectional view of a fiber provided by theinvention for optical and electrical transmission;

FIG. 5 is a plot of measured optical transmission as a function oftransmitted wavelength for the fiber of FIG. 4;

FIG. 6 is a plot of measured conducted electrical current as a functionof applied voltage for the fiber of FIG. 4;

FIG. 7A is a plot of the calculated photonic band structure for thefiber of FIG. 4;

FIG. 7B is a plot of the calculated HE₁₁ mode transmission spectrum as afunction of wavelength for the fiber of FIG. 4;

FIG. 8 is a schematic cross-sectional view of a fiber provided by theinvention for photodetection;

FIG. 9A is a plot of the measured current-voltage characteristics of thefiber of FIG. 8;

FIG. 9B is a plot of measured fiber resistance as a function ofillumination intensity for the fiber of FIG. 8;

FIG. 10A is a plot of measured fiber resistance as a function of fiberdiameter for a given fiber illumination, for the fiber of FIG. 8; and

FIG. 10B is a plot of measured fiber resistance as a function ofilluminated fiber length for a given fiber illumination, for the fiberof FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Fibers produced in accordance with the invention are three-dimensional,unsupported physical objects for which one dimension, defined as thelongitudinal dimension, is substantially larger than the other twodimensions, defined as the cross sectional dimensions, and arefabricated by, e.g., the production sequence 10 outlined in the flowchart of FIG. 1A. In a first example sequence step 12, conducting,semiconducting, and insulating materials are assembled into a fiberpreform. The preform includes the selected materials arranged in amacroscopic geometric configuration corresponding to, though notnecessarily equivalent to, the desired geometry of the fiber. Thepreform is characterized by a ratio of longitudinal to cross sectionaldimensions that is typically between about 2 and about 100. Onceassembled, the preform can be consolidated 14 in a next process step.The consolidation process, carried out under selected temperature andpressure conditions as described below, is employed when necessary forproducing intimate material interfaces in the preform and to ensureelement shape integrity throughout the draw process. The preform is thendrawn 16 into a fiber that preserves the cross sectional geometricconfiguration of the macroscopic preform while reducing preform featuresizes to smaller scales and producing extended fiber lengths of uniformcross section.

These striking dimensional shifts produced by the fiber drawing processare schematically illustrated in FIG. 2. A macroscopic assembly 18 ofconducting, semiconducting, and insulating materials is arranged as apreform 19 having a diameter, D on the order of about 10 mm to about 100mm and a length, L, on the order of centimeters, e.g., less than 100 cmor less than 50 cm. The structured preform 19 is then subjected toheating and deformation under fiber drawing conditions to produce afiber 20.

The resulting fiber 20 has a length, l, on the order of meters, e.g., 10m, 20 m, 50 m, 100 m, or longer, and a diameter, d, on the order ofbetween about 50 μm and about 2000 μm, resulting in alongitudinal-to-cross sectional ratio that can be above 1000, a lengththat can be more than 100 times greater than that of the preform, and adiameter that can be 10 times less than the diameter of the preform.Within the fiber, feature sizes on the order of 10's of nanometers canbe produced. The fiber drawing process of the invention therebypreserves the preform's element organization along its length whileforming intimate material interfaces and reducing element sizes to themicro- and nano-scale along extended fiber lengths.

As a result, the invention enables the production of extended-lengthfiber for combined optical and electrical transmission, as well asmicro-scale microelectronic and optoelectronic device operation alongthe fiber axial length and/or across the fiber's cross-section, withoutemploying wafer based microfabrication techniques. Macroscopic assemblyof a preform is in general convenient and does not require exoticprocess techniques or equipment. The invention is not limited to aparticular preform configuration or preform assembly technique. Anypreform configuration and preform assembly techniques that employconducting, semiconducting, and insulating materials that are compatiblefor co-drawing, as explained below, can be utilized.

FIG. 3A is a cross-sectional view of a first example fiber configuration22 that can be produced in accordance with the invention. While thisexample fiber is shown having a circular cross section, such is notrequired; any suitable cross sectional geometry, e.g., rectangular, canbe employed for the fiber as well as the elements included in thecross-sectional fiber arrangement. In the example fiber there isprovided a fiber core region 24 around which are provided a number oflayers 26, 28 ranging in number from one to any selected number, with afinal layer 30 at the surface. The fiber core region 24 can be providedas an air core or as a solid of conducting, semiconducting, orinsulating material. For example, the core region can be provided as asemiconducting or insulating material for optical transmission, with oneor more outer layers 26, 28 provided as conducting materials forelectrical transmission. One or more outer layers can also oralternatively be provided as semiconducting and insulating, for opticaltransmission, electrooptical device operation, and for electrical andoptical isolation, and the core region, in concert with one or moreouter layers, can provide optoelectronic device operation.

Also as shown in FIG. 3A, there can be included in the fiber 22 one ormore strands 32, and other geometric elements 34 of conducting,semiconducting, or insulating materials provided along the axis of thefiber. Such elements can be incorporated in one or more layers of thefiber and can be arranged around the fiber circumference. Thisarrangement can be extended as shown in the example configuration 37shown in FIG. 3B, with an array of strands 36 provided around a portionor the entire circumference of the fiber, and arranged in a desiredconfiguration with other fiber elements, e.g., surrounding a core region38, and optionally including one or more layers 39, 40 surrounding thecore and the strands 36. The invention is not limited to a particularconfiguration of strands or other elements. As shown in the examplefiber 31 of FIG. 3C, a multi-dimensional array of strands 35 can beprovided about a core region 33 or other selected preform element. Eachof the strands can be provided as conducting, semiconducting, orinsulating materials.

As shown in the example fiber configuration 80 of FIG. 3D, the materiallayers provided about a core region 82 or other fiber geometry can bediscontinuous, i.e., multiple distinct layer regions 84, 85, 86, 88, 89can be provided as an alternative to a continuous layer. Such distinctlayer regions can be provided adjacent to continuous layer regions 87,91, and can be provided at multiple locations across the fiber crosssection, and can be provided as any of conducting, semiconducting, orinsulating materials.

The invention contemplates fiber geometries in which arrangements ofconducting, semiconducting, and insulating elements are provided inintimate interfacial contact that enables optoelectronic deviceoperation. For example, as shown in FIG. 3E, there can be provided afiber configuration 44 in which a core region 46 is adjacent to and incontact with one or more regions that can be provided as conductingelectrodes 48. While the illustrated example shows two electrodes, it isto be recognized that any number of electrodes can be included. In thisexample, discussed in detail below, the core region 46 can be providedof a material, such as a insulator or semiconductor, that is selectedfor interaction with and/or control by the adjacent electrodes 48. Thecore region 46 and adjacent regions 48 thereby are provided asoptoelectronic device elements within the fiber.

The region 49 directly surrounding the electrodes can be provided as asuitable material, e.g., an insulating material. Indeed, the variouspreform regions and elements 46, 48, 49 can be provided of any ofconducting, semiconducting, and insulating materials. As shown in FIG.3E, additional layers of material 50, 52, 54 can be included around thecore region and electrodes. Additionally, one or more strands 56 ofmaterial can be incorporated in one or more of the layers, and one ormore electrodes 57 and/or other elements 58 can similarly beincorporated in the layers.

Turning to FIG. 3F, in a further example fiber configuration 60 providedby the invention, multiple cores 62 and/or strands and other elements66, 68, of arbitrary geometry, can be included in the fiber in a desiredarrangement across the fiber cross-section. Multiple core regions 62and/or strands can be provided with adjacent regions 64, e.g.,electrodes, in any desired arrangement, e.g., as one or more pairs ofelectrodes. The fiber strands 68 as well as the core regions can also beindividually axially surrounded by additional material layers 70, 72 orby other fiber elements. Finally, if desired, the fiber can include oneor more material layers that surround a material region 78 in which thevarious elements are provided. No particular symmetry is requiredbetween the various cores, strands, and other elements included in thefiber.

The examples of FIGS. 3A-3F demonstrate several particular advantages ofthe fiber and fiber drawing processes of the invention. Conducting,semiconducting, and insulating materials can be disposed in selectedconfigurations that enable the production of optical and electronicfunctional elements, e.g., optoelectronic device elements, within thefiber cross section. The fiber can include hollow regions or can becharacterized by an all-solid cross section, i.e., a cross sectionhaving no hollow regions. The fiber elements do not need to display thesymmetry of the fiber outer perimeter or if hollow, the fiber innerperimeter. The fiber can further be characterized by cylindricalasymmetry. The fiber can include elements that have a perimeter that isa closed loop which does not contain a characteristic center of a crosssection of the fiber.

There is no requirement as to the ordering of materials within the fiberof the invention, with the caveat that metal regions be geometricallyconfined in the manner described below. Having no such orderingrequirement, the fiber of the invention can provide one or more materialinterfaces and/or material composition discontinuities along a path,around a cross section of the fiber, that conforms to a fiber perimeter.In other words, along a path around the fiber cross section that is afixed distance from the fiber periphery at all points of the path,material interfaces and/or material composition discontinuities can beprovided. For specific geometries, such a path can be considered acircumferential path. This condition enables a wide range of fibergeometries, including, e.g., conducting electrodes at locations of thefiber cross section, across which can be sustained an applied voltage.

The fiber configuration of FIG. 3F can be adapted, however, with aselected symmetry and fiber element configuration. For example, as shownin the fiber configuration 65 of FIG. 3G, fiber elements can be arrangedwith a particular symmetry, e.g., in a two-dimensional sub-fiber arraythat can incorporate previously drawn fiber elements. In the examplefiber of FIG. 3G, a number, e.g., 1000, of individually drawn sub-fibers67, each having a selected cross section geometry that can besubstantially similar or dissimilar from the other sub-fibers, arearranged as a hybrid fiber. In the example shown, each sub-fiber 67 inthe hybrid fiber 65 is provided with a core region 62 and electrodes 64adjacent to and in contact with the core region, and having an outerregion 71 about the electrodes and core region. It is to be recognizedthat any geometry can be selected for each individual sub-fiber in thehybrid fiber, however. Additionally, the functionality of each sub-fiberin the hybrid fiber array can be selected to complement that of theother sub-fibers, and elements of the sub-fibers can be electrically orotherwise interconnected to form an integrated circuit hybrid fiber inwhich devices are provided across the various sub-fibers in the array.

The example fibers of FIGS. 3A-3G are provided to demonstrate the widerange of fiber configurations contemplated by the invention. Featuresshown in one or more of the example fibers can be selectively includedin a customized fiber configuration that provides desiredfunctionalities of optical transmission, electrical transmission, and/oroptoelectronic device operation. Because the fiber configuration enablesisolated arrangement of selected materials as well as intimate contactbetween materials, optical and electrical transmission can occursimultaneously yet separately, and in parallel with optoelectronicdevice operation. The materials selected for various fiber elements canbe customized for a given application, with conductor-semiconductor,conductor-insulator, and semiconductor-insulator interfaces occurringthroughout the fiber cross section and around the fiber circumference.

The direction of electron transmission, if such is accommodated by thefiber geometry, can coincide with or be counter to a direction of photontransmission through the fiber, if such is also accommodated by thefiber geometry. In general, the direction of electron and photontransmission can be longitudinal, i.e., along the fiber axial length,and/or radial, from a center region radially outward or from an outwardregion radially inward. The thicknesses of the materials included in agiven fiber configuration are therefore preferably selected based on theparticular fiber application and the desired direction of electronic andphotonic transmission, as discussed in detail below.

Whatever fiber configuration is selected, in accordance with theinvention the configuration includes conducting, semiconducting, andinsulating materials arranged as layers, regions, and/or elements, withselected material interfaces, that enable desired optical and/orelectrical functionality for the fiber. The conductivity of eachmaterial can be selected based on the functionality specified for thatmaterial. For example, suitable conducting materials can becharacterized by a conductivity greater than about 10² 1/Ω·cm; suitablesemiconducting materials can be characterized by a conductivity lessthan about 10² 1/Ω·cm but greater than about 10⁻¹² 1/Ω·cm; andinsulating materials can be characterized by a conductivity less than10⁻¹² 1/Ω·cm. This example characterization of materials highlights theparticular advantage of the invention in its ability to intimatelyincorporate materials having over ten orders of magnitude disparity inconductivity. It is to be recognized that there does not exist awell-defined, i.e., absolute, boundary in conductivity values betweenconductors, semiconductors, and insulators. These example values areprovided as a general aid in characterizing suitable conducting,semiconducting and insulating materials for optical and electrical fiberfunctionality.

As explained above, the various selected materials are first assembledin a macroscopic preform and then drawn to a final fiber geometry. Thethermal deformation conditions inherent in the fiber drawing processrequire that the conducting, semiconducting, and insulating materialsselected for a given fiber configuration be compatible for co-drawing.

For clarity of discussion, it is convenient to first describe theproperties of compatible semiconductor and insulating materials to beco-drawn in accordance with the invention. In general, it is recognizedthat materials which are amorphous and glassy are particularly wellsuited to be drawn from a preform into a fiber structure. The termamorphous here refers to a material morphology that is a continuousatomic network in which there is no repeating unit cell or crystallineorder; a glassy material typically is not easily crystallized at highprocessing temperatures. For many applications, it can be preferred toselect semiconducting and insulating fiber materials that are glassy toenable fiber drawing at a reasonable speed while self-maintainingstructural integrity and regularity. Such can be achieved with glassymaterials because the viscosity of a glassy material variesquasi-continuously between solid and liquid states, in contrast to acrystalline material. By employing a glassy material, it is ensured thatthe fiber structure will remain amorphous, i.e., not crystallize, whencycled through softening and drawing temperatures.

Considering the viscosities of candidate glassy semiconducting andinsulating materials, suitable materials for co-drawing are those havingcompatible viscosities at the fiber drawing temperatures of interest.More specifically, the materials should both be above their respectivesoftening points at an overlapping draw temperature to enable theirco-drawing. Precise viscosity matching between fiber materials is notrequired; specifically, the materials need not have the same viscosityat the draw temperature, but rather all should flow at that commontemperature. It is further understood that for some materialcombinations, high viscosity in one or more materials that comprise themajority of the volume of the fiber preform is sufficient to enablestructural integrity of all co-drawn materials. Suitable materialsadditionally are preferably characterized by good surface adhesion andwetting in the viscous and solid states without cracking even whensubjected to thermal quenching.

There have been identified in accordance with the invention a class ofinsulating materials, namely, amorphous thermoplastic polymericinsulating materials, that are particularly well-suited to the fiberco-drawing process of the invention. High glass-transition-temperaturepolymeric insulators are an example of such; a wide variety of amorphoushigh glass-transition-temperature polymer materials are available andcan be processed with a range of techniques to form various materialconfigurations that are characterized by excellent mechanical toughness.Examples of high glass-transition-temperature polymers that can beemployed include poly-ether imide (PEI), poly-sulfone (PS), poly-etherether ketone (PEEK), and poly-ether sulfone (PES).

There also can be employed as an insulating material liquid crystalpolymers (LCP's), low glass transition polymers such as poly-methylmethacrylate (PMMA), polycarbonate (PC), poly-ethylene (PE) and othersuch thermoplastic polymers. Poly-tetrafluoroethylene (PTFE or Teflon™)and other fluorinated polymers or copolymers can also be employed inconfigurations in which their characteristically poor surface adhesionproperties can be accommodated. While it is preferred that amorphouspolymer materials be employed, it is also recognized that somesemicrystalline polymers, e.g., branched PTFE, can be employed. Anecessary condition for any suitable polymeric material is that thereexist a fiber draw temperature at which the polymer can be drawn into afiber at a reasonable speed, e.g., greater than about 1 mm/minute,without decomposition.

Considering candidate semiconductor materials for the fiber co-drawingprocess of the invention, amorphous semiconductors are preferred, giventheir low glass transition temperatures and stability with respect tooxidation. Amorphous semiconductors are also preferred for their goodwetting properties, defined by the contact angle between thesemiconductor and polymer materials at the draw temperature; a contactangle of less than about 150 degrees can be preferred. Further,amorphous semiconductors generally are characterized by a viscosityvalue that is similar to that of the polymers described above at polymerdraw temperatures. Both organic semiconductors, such as PPV, or polythiophene, as well as inorganic semiconducting materials can beemployed.

The class of semiconducting chalcogenide glasses are particularlywell-suited to the co-drawing process of the invention. Chalcogenidesare high-index inorganic glasses that contain one or more of thechalcogen elements of sulfur, selenium, and tellurium. In addition tothe chalcogen element, chalcogenide glasses can include one or more ofthe following elements: boron, aluminum, silicon, phosphorus, sulfur,gallium, germanium, arsenic, indium, tin, antimony, lithium, thallium,lead, bismuth, cadmium, lanthanum, and the halides fluorine, chlorine,bromide, and iodine. There is a very wide variety of differentcompositions within the family of chalcogenide glasses and thus theproperties of a given composition can be tailored through compositionaladjustment. For example, a composition of (As₄₀Se₆₀)_(1-x)Sn_(x) can beemployed to obtain a desired characteristic.

For many applications, the semiconducting material is best selectedbased on its material characteristics for enabling photonic conductionand optoelectronic device operation. For example, the amorphoussemiconducting material can be compositionally tailored to achievedesired optical, thermal, and/or mechanical properties. In one examplescenario, the semiconducting material is selected in combination withthe insulating material to produce a multilayer photonic bandgapstructure for conduction of photons through a hollow fiber core aroundwhich are provided alternating semiconducting and insulating layers.Such a configuration is described in U.S. patent application Ser. No.10/733,873, entitled “Fiber Waveguides and Methods of Making Same,”filed Dec. 10, 2003, issued Sep. 18, 2007, as U.S. Pat. No. 7,272,285,the entire contents of which is hereby incorporated by reference. Inthis example, chalcogenide semiconducting materials such as As₂Se₃;(As₂Se₃)_(x)M_(1-x), where M is In, Sn, or Bi; (As₂Se₃)_(1-x)Sn_(x);As—Se—Te—Sn, or other chalcogenide materials are employed with PES, PEI,or other suitable amorphous polymer to produce the desired bandgapstructure. It is to be recognized that a wide range of polymers can bepaired for co-drawing with a chalcogenide material; e.g., both high andlow glass-transition-temperature polymers can be employed in conjunctionwith low glass-transition-temperature chalcogenide glasses.

The conducting material to be employed in the fiber of the invention isselected based on its compatibility for co-drawing with the selectedsemiconducting and insulating materials. At a selected fiber drawtemperature, the selected conducting material should be molten orsufficiently ductile to enable thermal deformation. For manyapplications, it can be preferred to employ a conducting material havinga melting temperature that is below a desired fiber draw temperature. Itadditionally is preferred that the conducting material sufficiently wetthe surfaces of the semiconductor and insulating materials such that thecontact angle between the conducting material and these materials isless than about 150 degrees, at the fiber draw temperature, for the caseof a bare-surfaced conducting material, without inclusion of an adhesionpromoter.

Given a selection of a high glass-transition-temperature polymericinsulating material and a chalcogenide semiconducting material, a lowmelting-temperature metal or metal alloy can be a preferable conductingmaterial selection. For example, tin, indium, bismuth, cadmium, lithium,or other low melting-temperature metal is particularly well suited forthe material trio, as well as Sn-based or other selected alloys. Inaddition, a selected metal alloy can be synthesized to provide desiredmelting temperature, electrical conductivity, and other properties. Forexample, Sn—Ag, Sn—Sb, Sn—Cu, and other alloys can be employed. Further,there can be employed suitable amorphous glassy metals, or othersuitable metal composition.

With these considerations and examples, it is to be understood that someexperimental verification may be required to confirm the co-drawingcompatibility of various candidate materials. Once the drawingtemperature of each material of interest is determined, and assumingthat the materials can be drawn within a common temperature range, itcan be prudent to examine the viscosities of the materials across theselected drawing temperature range to ensure that the viscosities arecompatible. As stated above, it is not required that the viscosities ofthe various materials be the same at the fiber draw temperature, butrather that all materials should at least flow at the draw temperature,with conducting materials preferably molten at the draw temperature.Also, as stated previously, it is understood that it can be preferredthat the material which comprises the majority of the volume of thefiber preform be characterized by the highest viscosity.

For example, a reasonable criteria for a material trio including a highglass-transition-temperature polymer, a chalcogenide semiconductor, anda metal is that all materials have viscosities lower than about 10⁸Poise at the selected draw temperature, with metals preferably beingmolten at the selected draw temperature. If, e.g., the polymer materialconstitutes the majority of the fiber preform volume, then a polymerviscosity of between about 10¹ Poise and about 10⁸ Poise can beacceptable, with a viscosity of between about 10⁴ Poise and about 10⁷Poise preferred, all at the fiber draw temperature. In this examplescenario, a viscosity of less than about 10⁸ can be acceptable for thesemiconducting and conducting materials included in the fiber preform,with a viscosity of less than about 10⁶ preferred.

It is to be recognized that the fiber of the invention is not limited toa single conducting material selection, a single semiconducting materialselection, or a single insulating material selection. Rather, any numberof co-drawing compatible materials from the three material classes canbe employed as necessary for a given fiber configuration andapplication. In addition, distinct material layers, regions, andelements can be included all of distinct thicknesses, dimensions, andcomposition. For example, various materials can be included to tailoroptical signal transmission rates. In one such scenario, the inclusionof optical defect layers adjacent to optical transmission layers, andthe tailoring of the thicknesses of such layers, can be employed toachieve a photonic propagation rate that is commensurate with the rateof electron propagation through other elements of a fiber.

Similarly, various conducting materials can be included with theirdimensions tailored for a specific operation. For example, given a metallayer or strand incorporated into a fiber for electron conduction, themetal is preferably of sufficient thickness to achieve meaningfulelectrical conductivity for a given application, at reasonable appliedvoltage biases.

The thickness of the metal is preferably selected based on a givenapplication and the direction of required electronic conductivity.Recall that the resistance, R, in ohms, of a conductor is proportionalto the conductor resistivity, ρ, length, l, and is inverselyproportional to the conductor cross sectional area, A, as R=ρl/A. Thusif an electrical potential difference is applied across a metal layer ofthe fiber in the radial direction, for radial conduction, a very thinmetal layer can be sufficient to conduct large currents, while ifconduction is to be in the axial direction, along the fiber length, thena metal layer as thick as 25 microns may be required for reasonableconduction along, e.g., a 10 m fiber section. In general, whateverconductor configuration is selected, it preferably is characterized by aresistance per unit length of less than about 1 KΩ/cm to enableeffective electronic conduction. Various conducting materialcompositions and geometric combinations can be employed to tailor theconducting properties for a given application.

Assembly of materials into a fiber preform is carried out employingprocesses that are compatible with the selected materials to producedesired material configurations based on the considerations describedabove. No particular preform assembly technique is required by theinvention. Rather, a range of techniques can be employed to produce apreform having a configuration corresponding directly to the desiredpost-draw fiber.

In accordance with the invention a variety of preform elements can beprovided and/or produced separately for incorporation together into apreform arrangement. Considering first conductive materials,commercially available rods, strands, foils, sheets, and other articlesof conducting material can be employed. Thermal evaporation, E-beamevaporation, sputtering, chemical vapor deposition (CVD) and otherphysical deposition techniques can be employed for coating preformelements with a conducting material layer or layers. It is to berecognized, however, that depending on a particularly selecteddeposition technique and the deposition parameters, not all depositedfilms may be compatible with a fiber co-drawing process; e.g., thedeposited conducting material must be sufficiently thick as well asductile to accommodate the drawing process.

Whatever conducting material geometry is employed, if the conductingmaterial is a metal or metal alloy that will melt at fiber drawtemperatures, then in accordance with the invention, the metal or alloyis arranged in the preform such that it is confined geometrically bymaterials in the preform that will not melt at the draw temperatures.This metal confinement ensures that the draw process retains the desiredmetal configuration in the fiber even while the metal is in a fluidstate.

In addition, it is recognized in accordance with the invention thatconducting materials can oxidize readily at elevated temperatures,including preform consolidation and fiber draw temperatures. Oxidizedconducting materials may not melt or may flow nonuniformly, resulting innonuniform or even inoperable conducting elements in the drawn fiber. Toeliminate this condition, it can be preferred in accordance with theinvention to inhibit and/or remove oxide from conducting elementsurfaces for various preform geometries.

The invention provides a range of techniques for inhibiting oxidizedconducting materials in a drawn fiber. In a first example technique, anantioxidizing, or oxide inhibiting, agent that preferably is a surfacewetting promoter is incorporated into the preform at interfacessurrounding the conducting material, e.g., surrounding metal elements inthe preform. This can be achieved by, e.g., physically applying anoxidation inhibitor to the conducting material surfaces during thepreform assembly. A particularly well-suited oxidation inhibitor is aflux; fluxes in general are synthetic carboxylic acid-containing fluidsor natural rosin fluxes. These compounds serve to enhance and promotethe wetting of the preform materials by the metal or other conductingmaterial so as to prevent capillary breakup of the conducting material.This enables the use of conducting materials that may not normallyexhibit the required surface wetting condition. Example suitable fluxesinclude Superior No. 312 flux, or Superior 340 flux, both from SuperiorFlux and Mfg. Co., Cleveland, Ohio. The flux can be applied directly tothe conducting material surfaces, and can alternatively or in additionbe applied to surfaces of other materials that in the preformconfiguration are to be adjacent to conducting material surfaces.

In a further technique for inhibiting oxidized conducting elements in adrawn fiber, an oxidation inhibitor can be applied to one or morepreform elements by adding it to the elements. For example, an oxidationinhibitor can be added to a polymer material that is to be locatedadjacent to a conducting material element. The oxidation inhibitorconstituent preferably segregates to or is naturally located at thesurface of the polymer for application interaction with adjacentconducting materials. Alternatively, a polymer, semiconductor, or othermaterial that itself has oxidation inhibition or oxide growthsuppression properties can be selected for use in the preform adjacentto conducting elements. Oxidation inhibiting and/or growth suppressionbuffer layer materials can also be included between a conducting elementand an adjacent material. Whatever oxidation inhibition technique isemployed, it is preferred that the oxide inhibitor does not decompose atthe preform consolidation temperature or the fiber draw temperature.

Considering the need to encapsulate metal preform elements and inhibitoxide of such elements, in one example for encapsulating metal strands,polymer-coated metal strands are produced from commercially-availablemetal wires. In one such scenario, Sn wires, e.g., 5 mm in diameter arecoated with a layer of flux, such as that described above, and thenwrapped with a layer of PES film, e.g., 7.5 mm-thick PES filmcommercially available, e.g., from Westlake Plastics Co., Lenni, Pa.Alternative wire coating techniques can be employed, such as dipcoating. The ends of the wrapped wires are then coated with a polymermaterial, e.g., by dip-coating. For this application, a polymersolution, e.g., 20% PES, 80% N,N-Dimethylacetamide, can be employed. Thepolymer solution is then solidified on the wire by heating thestructure, e.g., at 180° C., or by subsequent consolidation of thestructure. Consolidation of the polymer-wrapped wires can be preferredfor ensuring intimate contact between the metal and polymer materials,and can be carried out in a vacuum oven at, e.g., 260° C.

The heating step or consolidation process results in the polymersolution being solidified and the wires thereby encapsulated with apolymer layer. The polymer-wrapped wires can at this point be drawn toform metal strands of a desired diameter, if a reduced diameter isdesired for a given application. For the example of PES-coated Sn wires,a draw temperature of about 305° C. in a vertical tube furnace producespolymer-coated metallic strands having an outer diameter between about500 μm and about 1.5 mm, depending on draw conditions. The metallicstrands can then be incorporated in a fiber preform arrangement in themanner described below. It is to be recognized, however, that if desiredfor some applications, as described below, metallic wires or otherelements can be drawn to a desired diameter without encapsulating theelements in a polymer or other insulating material.

Considering insulating fiber preform elements, due to the relative easeof preform assembly and drawing of polymer materials, compared withother glassy insulating materials, polymeric insulating materials can bepreferred for many fiber applications. Polymeric insulating materialscan be readily obtained commercially or produced in a desiredconfiguration. For example, commercially available polymer rods, tubes,sheets, and films from, e.g., Westlake Plastics Co., can be employed.Polymer rods and tubes can also be produced by thermal consolidation ofa rolled polymer film. Polymer layers can be produced by chemical vapordeposition techniques such as plasma enhanced chemical vapor deposition,by spin-coating, dip-coating, as described above, by roll-casting,extrusion, and other techniques. Liquid polymer can be applied, asdescribed above, for coating preform core materials, strands, wires,rods, layers of other material, and preform elements.

Chemical and physical deposition techniques can be employed forproducing non-polymeric insulating material preform elements. Theinvention does not limit insulating materials to polymeric materials. Solong as a candidate insulating material is characterized by a morphologythat is compatible with fiber drawing, such can be employed in additionto or as an alternative to polymeric materials.

Similarly, chemical and physical deposition techniques can be employedfor producing amorphous semiconducting material preform elements. Asexplained above, for many applications, chalcogenide glasssemiconductors can be preferred for their co-drawing compatibility withpolymeric insulators. Rods, tubes, sheets, films, and othersemiconducting structures can be employed in the fiber preform. A widerange of semiconducting glass structures can be obtained commercially,e.g., from Alfa Aesar, Ward Hill, Mass., and also can be synthesized asa particularly desired composition and geometry.

For example, in accordance with the invention, chalcogenide glassstructures can be chemically synthesized using sealed-ampoule meltquenching techniques. In one example scenario, pure elements such as Asand Se are placed in a quartz tube under a nitrogen atmosphere. The tubeis initially maintained open at one end. A vacuum line is connected tothe open end of the tube and the tube is preheated under vacuum to meltthe elements and remove trapped gasses and surface oxide. Heating to330° C. for one hour at a heating ramp rate of about 1° C./min andthereafter cooling to room temperature at a ramp down rate of 1° C./minis sufficient. An oxygen gettering agent such as Mg can be added to thetube to reduce the partial pressure of oxygen within the tube.

The tube is then sealed under a vacuum of, e.g., 10⁻⁵ Torr, using, e.g.,a high-temperature torch. The sealed tube is then heated in a rockingfurnace for physically mixing the elements during a prescribed heatingschedule corresponding to the elements included. For example, the As—Semixture can be heated to 800° C. at a rate of about 2° C./min, whileheld vertical, for twenty four hours, and then rocked for six hours toincrease mixing and homogenization. The glass liquid is then cooled,e.g., to 600° C., in the furnace, and then quenched in water.Subsequently, the mixture is annealed for one half hour to the glasstransition temperature, e.g., about 180° C., before being cooledgradually to room temperature. Using this synthesis technique,mechanically strong semiconducting structures can be fabricated as,e.g., rods, tubes, and other structures. Once the glass is synthesized,it is no longer sensitive to oxygen at room temperature. It thereforecan easily be handled in ambient atmosphere for incorporation into apreform or employed for further processing.

In addition to conducting, semiconducting, and insulating preformelements, sacrificial elements can be included in a preform to aid indefining a preform shape, and then removed prior to drawing of thepreform into a fiber. For example, quartz tubes or rods can be includedin a preform at locations for which a hole is desired, and thenchemically etched away after consolidation of the preform. Similarly,Teflon™ rods or tubes can be included in a preform and mechanicallyremoved from the preform after consolidation. This technique provides aparticularly elegant method for defining gaps and spaces in a preformassembly prior to fiber drawing.

With preform building blocks like the examples described above, and withsuitable fabrication processes, like those described above, a wide rangeof preform geometries can be assembled for enabling optical andelectrical functionality, including transmission and device operation,in a final fiber structure. Semiconducting, insulating, or conductingrods, strands, and other geometric elements can be coated with selectedmaterial layers. Various material layers can be applied in any order,with the caveat that metals be geometrically confined by materials thatwill not melt at the draw temperature. Drilling, casting, injectionmolding, or other techniques can also be employed for defining thegeometric relationship between material elements in a preform layer orregion.

Considering now specific preform assemblies for producing fibergeometries like those corresponding to FIGS. 3A-3G, as described above,wires, strands, or rods of conducting material can be arranged in apreform. The conducting elements can be positioned on preform layers byemploying, e.g., a liquid polymer solution that operates to “glue” theelements, particularly polymer-coated elements, to a layer or otherelement. Referring to FIGS. 3A-3C, individual strands or an array ofstrands can be positioned around an inner layer or other element by thistechnique. Given a PES insulating layer composition, a polymer solutionof 20% PES, 80% N,N-dimethylacetamide, available from Alfa Aesar, asdescribed above, can be employed as both an end-coating strand mediumand the liquid attachment medium. As explained above, during a heatingstep or a thermal consolidation process, the liquid polymer solutionsolidifies to a solid PES region that is an integral part of the preformand fiber.

It is to be recognized that conducting strands, wires, rods, or otherelements are not required to be covered with a polymer coating, but suchcan be a convenient technique for geometrically confining the conductingmaterial within the preform assembly. Alternatively, uncoated conductingstrands can be positioned around a layer, e.g., a polymer layer, withpieces of polymer film cut and fit between each strand, and a layer ofpolymer applied over the array of strands. Other materials can beemployed for confining the metal so long as the materials cooperate witha desired fiber functionality and can geometrically confine the metalduring a draw process.

For many applications, where a high glass-transition-temperature polymeris employed as an insulating material, it can be particularlyadvantageous to employ layers of polymer material in assembly of apreform structure. For example, for the photonic bandgap structuredescribed previously, alternating layers of semiconducting andinsulating materials can be produced by depositing a semiconductor layeron one or both sides of a polymer film and then rolling the film into acylindrical multilayer structure. In addition, a polymer film can berolled around individual preform elements, such as conducting strands,as described above, and further can be rolled around assemblies ofelements like those shown in FIGS. 3A-3G to produce insulating fiberregions and outer layers.

Deposition of a semiconductor layer on one or both sides of a polymerfilm can be accomplished by thermal evaporation, chemical vapordeposition, sputtering, or other suitable deposition technique. Where asemiconductor such as a chalcogenide glass has been synthesized, e.g.,by the chemical synthesis process described above, conventional thermalevaporation of a synthesized source material onto a polymer film can bea particularly convenient deposition technique. It is preferred that thepolymer film be highly uniform in surface quality and thickness and becleaned, e.g., with an alcohol, prior to the deposition process. Thermalevaporation can be carried out with conventional hot filamentevaporation techniques, preferably at a pressure below, e.g., about 10⁻⁴Torr. A conventional vacuum evaporator, e.g., a Ladd Research IndustriesModel 30000, can be employed. If desired, both sides of a polymer filmcan be coated with a selected material.

In order to assemble a layered preform structure of the polymer film anda material deposited on the film in the manner just described, thecoated polymer film can be wrapped, or rolled, around a mandrel or otherpreform structure a number of times to produce a desired number oflayers. For example, in production of a photonic bandgap structure foroptical transmission, a PES film coated with As₂Se₃ can be rolled anumber of times to produce a structure with 20 or more alternatingsemiconducting and insulating layers. In this scenario, the PES film andthe As₂Se₃ layer thicknesses are selected specifically to achieve amaximal photonic bandgap at a desired wavelength of photon transmission.The desired thicknesses of the layers in the final fiber structuredictate the thicknesses of the materials in the preform, based on theselected draw conditions, as explained in detail below.

Where the photonic bandgap structure is to conduct photons through acentral hollow core surrounded by the bandgap materials, thesemiconductor-coated polymer film can be rolled around a sacrificialpreform such as a glass rod, hollow glass tube, or Teflon™ rod or tube,or other structure that can be removed from the preform prior to thefiber drawing step. Where the coated polymer film is to be employed inan alternative configuration, the coated film can be rolled around otherselected preform elements, e.g., polymer, semiconductor, or conductingrods, or other layers of preform materials, including metallic foils,semiconducting layers, or other preform elements. This enables thelayered structures shown in FIGS. 3A-3G.

Also as shown in FIGS. 3A-3G, conducting elements can be configured in apreform directly adjacent to semiconducting elements, e.g., aselectrodes 48 adjacent to a semiconducting core 46 shown in FIG. 3E. Inone example preform assembly technique, the use of polymer films can beparticularly advantageous for producing this structure. Considering ascenario in which a semiconductor core is to be contacted by metalelectrodes, the semiconductor core can be obtained commercially or bechemically synthesized in the manner described above.

A PES or other polymer film is then provided, having any desired length,a width that corresponds to the length of the semiconductor rod,preferably slightly longer than the rod, and a desired thickness, e.g.,125 μm in thickness. It is preferred to clean the film, e.g., withalcohol, and bake the film, e.g., at 150° C. for 3 hours, to remove thealcohol.

The electrodes can be formed in conjunction with the polymer film using,e.g., tin foil, of a desired thickness, e.g., between about 25 μm andabout 1 mm in thickness. Suitable foils can be commercially obtained,e.g., from Goodfellow Corporation, Devon, Pa., or can be produced by,e.g., pressing a metal rod to the desired foil thickness. The foil ispreferably cleaned and dried in the manner of the polymer film.Additionally, it can be preferred to coat the foil with an oxideinhibitor, e.g., a flux, as described above.

The conducting electrodes are shaped by cutting the foil into desiredelectrode geometries. If, e.g., the electrodes are to be configured asrectangles extending along the fiber longitudinal axis, then rectangulartin foil pieces are correspondingly cut. It can be preferred to cut foilpieces that are slightly shorter than the semiconductor rod length toenable geometric confinement of the foil in the preform in the mannerdescribed below. The width of the foil pieces is set based on theparticular functionality desired for the electrodes, e.g., a 5 mm-widefoil piece can be employed.

The foil pieces are assembled in a preform configuration by removingsections of the polymer film at film locations corresponding to theelectrode geometry and the desired placement of the electrodes. The filmsections can be removed through their entire thickness or a portion ofthe film thickness. Considering the placement of two electrodes equallyspaced around a semiconductor rod, and given an electrode width, w, andthe rod perimeter, P=πd, where d is the rod diameter, then the twoelectrodes are to be spaced a distance (P−2w)/2 apart on the film.Similar computations can be made to position any number of electrodes ina film to achieve a desired electrode configuration in a final fibergeometry. The foil electrodes are inserted into the film at thelocations at which the film was removed, and if desired, an additionallayer of film can be overlayed. The film-electrode assembly is thenrolled around the rod or other element, taking care that the electrodefoil material is contacting the rod.

If the polymer film is thinner than the metal foil or other conductingelement, then it can be preferred to employ several layers of polymerfilm, with each of the layers having an appropriate amount of materialremoved at desired electrode locations. Alternative to the use of apolymer film, one or more polymer tubes or other structures can beemployed for supporting electrode elements to be incorporated into apreform. For example, sections of a polymer tube can be removed forpositioning foil pieces in the tube, with the tube then slid over a rodor other preform element. For many applications, the use of a polymertube can be preferred for ease in positioning the metal electrodes andassembling the polymer-electrode configuration on another preformelement.

Extended sections of foil or other conducting material can be applied topolymer films or other materials to be wrapped around preform elementsin any geometry so long as confinement of conducting materials isachieved by the arrangement. For example, in thesemiconductor-metal-insulator arrangement just described and shown inFIG. 3E, the electrodes 48 are radially confined between asemiconducting material 46 and an insulating material 49. In the examplefiber of FIG. 3D, discrete conducting layer segments 84, 85, 86,88, 89are confined between adjacent material layers 87,91, as well as materialregions located between each conducting segment. Such confinement can beenabled by the polymer film-foil assembly described above, with regionsof polymer film removed at desired conduction segment locations andmetal foil inserted into the film regions.

Further, it was suggested just above that the electrode foil not extendthe entire length of a polymer film to ensure that the polymer confinesthe foil at the longitudinal ends of the electrodes. If this is not thecase, then it is preferred that an encapsulating material be applied atthe preform ends. As described previously, a particularly convenienttechnique for such encapsulation can be the application of a liquidpolymer solution that is dried during a subsequent heating or thermalconsolidation step.

Beyond conducting elements, additional preform elements can be added andarranged. For example, as shown in FIG. 3E, additional material layers50, 52, 54, possibly including strands or other elements 56, 57, 58, 66,68, cores 62, or other elements, can be incorporated in the preform. Asshown in FIG. 3F, this arrangement can be extended to produce a preformthat includes multiple rods 62 in a selected configuration, e.g., havinga selected number of adjacent electrodes 64. Layers of polymer film canbe wrapped around each rod-electrode combination, and employing, e.g., aliquid polymer solution like that described above, can be attachedtogether and to, e.g., a central polymer support structure.

This preform assembly process can be extended to the arrangement offiber elements in a preform for producing a hybrid fiber array like thatshown in FIG. 3G. In this process provided by the invention, one or morepreforms are assembled and drawn into one or more sub-fibers. Thesub-fibers are then cut into fiber sections that are arranged in adesired array or other hybrid arrangement. This hybrid arrangement isthen drawn into a hybrid fiber including the array elements.

Each of sub-fiber elements included in the hybrid fiber array can beproduced in the manner described above, with various conducting,semiconducting, and insulating material elements arranged in a varietyof selected preforms. The preforms are then drawn to sub-fibers, in themanner of the drawing processes described below. For the example fibershown in FIG. 3G, a preform including a rod, e.g., a semiconducting rod,such as an As₂Se₃ rod, is arranged with metallic electrodes 64, e.g., Snelements, provided adjacent to and in contact with the rod, withpolymeric material 71, e.g., PES, about the rod and electrodes. Theprocess described above for assembling Sn foil into apertures cut in apolymer film can here be employed. With the preform complete, such isthen drawn into a sub-fiber of e.g., about several meters in length andcut into a number of sub-fiber sections, e.g., 1000 sections, eachsection having a length of, e.g., 15 cm.

In assembly of a preform for the hybrid fiber, each of the cut sub-fibersections is arranged, e.g., in an array. In one example technique forproducing such an array, the sub-fiber sections are inserted inside ahollow sacrificial tube element, e.g., a quartz or Teflon™ tube. Becausein this example each sub-fiber section includes electrodes that extendto the end of the sub-fiber section, it can be preferred dip the tube ofsub-fibers into, e.g., a liquid polymer solution to coat the ends of theelectrodes such that they will be geometrically confined when the hybridfiber is drawn. The arrangement is then consolidated, in the mannerdescribed below, if necessary, and the sacrificial quartz tube removedor etched from the preform by, e.g., a liquid HF etch process. As aresult of a consolidation process, the arrangement of sub-fiber sectionsare fused together as a unitary hybrid structure and the electrodes arecoated with a polymer at the sub-fiber ends.

The hybrid preform structure can then be drawn into a hybrid fiberstructure under the draw conditions described below. In the currentexample of sub-fiber elements, each sub-fiber section in the fiber arrayis, e.g., about 400 μm in diameter after its first drawing. The hybridfiber drawing process further reduces the diameter of each sub-fiber;for example, given a drawdown reduction factor of 20, the diameter ofeach 400 μm-diameter sub-fiber section is reduced to 20 μm. Thus, as aresult of the dual draw processes employed in this method, very minutefiber features are produced in each of the sub-fiber sections includedin the hybrid fiber.

In addition to the preform assembly techniques described above, theinvention contemplates drilling, casting, molding, and other techniquesfor producing a preform. For example, holes can be drilled in a polymerbody and conducting or semiconducting strands or other elements fittedinto the drilled regions. Any preform assembly technique thataccommodates all of conducting, semiconducting and insulating materialsin an arrangement that enables co-drawing of the three materials can beemployed.

Depending on the selected preform assembly technique and resultingarrangement, it can be preferred to thermally consolidate an assembledpreform prior to the fiber drawing process. Consolidation is a processwhereby under heat and vacuum conditions one or more of the preformmaterials are caused to fuse together, with air pockets in the preformbeing substantially eliminated. This results in a preform assembly thatcan produce intimate interfacial contact between adjacent materiallayers in the final fiber, and provides the preform withself-maintaining structural stability during the fiber draw process.

The specific conditions of the consolidation process are selected basedon the particular materials incorporated into a given preform. If, e.g.,a high glass-transition-temperature polymer is employed in the preform,then the consolidation temperature preferably is above the glasstransition temperature of the polymer. The preform is maintained at theelevated temperature for a time sufficient to cause the polymer to fuseto adjacent elements included in the preform; the temperature isselected based on the preform diameter and its materials. Given apreform including PES polymer elements, As₂Se₃ semiconducting elements,and Sn metal elements, a consolidation temperature of between 250°C.-280° C., e.g., about 260° C., at a pressure of about 10⁻³,sufficiently consolidates the structure.

For most consolidation temperatures, metal preform elements will bemelted during the consolidation process but confined to their intendedgeometries by the arrangement of confinement layers described above.Depending on the consolidation temperature, semiconducting preformelements may soften or may remain solid. The inclusion of at least onematerial that can fuse to adjacent materials during consolidation is allthat is required. In the PES-As₂Se₃—Sn example given above, theconsolidation temperature is set to enable softening and fusing of thePES polymer to adjacent preform elements.

It can be preferred to carry out the consolidation process in a verticalrotating zone refinement furnace. In such a furnace, the preformlongitudinal axis is held vertically and a zone refining heating processis carried out along the preform length. Preferably the consolidation isconducted from the preform bottom upward through the preform to its top.The heating time for each incrementally consolidated preform sectionalong the preform length is determined based on the preform diameter andmaterial elements as explained above.

As explained above, in construction of a preform there can be includedone or more sacrificial elements that are incorporated in the preformsolely to define spaces to be provided in a final fiber geometry. Forexample, a mandrel, rod, or tube can be included in a preform where ahollow fiber core or other region is desired. If a sacrificial elementis included in a preform, it is preferred that the consolidation processbe carried out at a temperature below the glass transition temperatureof that element, so that structural integrity of the sacrificial elementis maintained during the consolidation process and the preform does notcollapse on itself.

For many preform material arrangements, a sacrificial element can beconstructed that can withstand reasonable consolidation temperatures andpressures and can easily be removed from the preform afterconsolidation. For example, Teflon™ tubes, rods, or other elements canbe readily incorporated into and removed from a preform. Any materialthat exhibits poor surface adhesion and can withstand the consolidationprocess is a good sacrificial element material. It is preferable toremove the Teflon™ or other sacrificial element immediately after theconsolidation process, while the preform is hot and slightly expanded.This enables ease of removal. Once the preform cools and correspondinglyshrinks, it can be difficult, if not impossible, to remove the elementby simple mechanical force.

Alternatively, sacrificial elements which can be removed from aconsolidated preform by chemical etching can be employed. For example,glass, quartz, or other etchable materials that can withstand theconsolidation process can be employed. In such a scenario, after theconsolidation process, the preform is exposed to a chemical etchant thatselectively attacks the sacrificial elements. For example, hydrofluoricacid or other acid bath can be employed for wet chemical etching ofsacrificial elements. Dry etch techniques, e.g., plasma etch techniques,can also be employed if such can be adapted to contact and selectivelyattack the sacrificial materials in a preform.

Once a preform has been consolidated, if necessary, and sacrificialelements removed from the preform, drawing of the preform into a fibercan proceed. Fiber drawing can be carried out in a fiber draw tower orother suitable draw apparatus. In such an apparatus, a top preformdownfeed mechanism is provided for holding an end of the preform andlowering the preform into a furnace. It can be preferred to employ avertical draw furnace enabling three temperature zones, namely, top,middle, and bottom temperature zones. Below the furnace is provided acapstan with spooler for spooling the drawn fiber. Measurementequipment, e.g., a laser diameter monitor, from Beta LaserMike, Dayton,Ohio; fiber tension measurement devices, e.g., Model SM9649P, fromTension Measurement, Inc., of Arvada, Colo., and other monitoringequipment can be included.

The draw furnace temperature zones, preform downfeed speed, and capstanspeed are selected based on the preform materials and configuration toenable co-drawing of preform conducting, semiconducting, and insulatingmaterial elements into a desired fiber configuration. The top furnacezone temperature is selected to cause the preform materials to softenbut not flow. The middle furnace zone temperature is selected as thedraw temperature, to cause the preform to flow into a fiber form. Asexplained above, the draw temperature is selected to be above the glasstransition temperature of the insulating and semiconducting materials,and for most material combinations, will be above the meltingtemperature of the conducting material. If an excessively high drawtemperature is employed, the preform will catastrophically deform, whilean excessively low draw temperature will cause preform distortion andexpansion. The structural arrangement of the preform must be preservedat the draw temperature.

It is therefore to be recognized that some experimental testing of drawtemperatures can be required for a given preform assembly. As explainedabove, a reasonable criterion for polymer, metal, and chalcogenidematerial draw temperatures is that all materials have a viscosity lowerthan about 10⁸ Poise at the draw temperature and that the metal bemolten at the draw temperature. Given a preform of PES polymericinsulating elements, As₂Se₃ semiconducting elements, and Sn conductingelements, a top zone temperature of between about 180° C.-250° C., e.g.,190° C.; a drawing zone temperature of between about 280° C.-315° C.,e.g., 300° C.; and a bottom zone temperature that is unregulated, andtherefore at, e.g., about 100° C., due to proximity to the draw zone,can be employed for successfully drawing the preform into a fiber.

For many applications, it can be preferred to ensure uniform heating ofthe preform during the drawing process. A uniformly heated furnaceemploying, e.g., distributed filament heating, is particularly wellsuited for the drawing process. It is further preferred that the preformbe maintained laterally centrally in the drawing temperature zone. Ifthe preform temperature distribution becomes nonuniform due to lack offurnace temperature control or lateral misalignment of the preform as itpasses downward through the drawing zone, there could be produced localpreform regions of differing temperature and differing viscosity. Localviscosity fluctuations in the preform could produce a capillary effectin which material, particularly molten metal, flows to other preformregions, and distorts the intended fiber geometry. The physicalconfinement of metal elements described above can be important forinhibiting such a condition, but in general, uniform preform heating ispreferred for preserving an intended fiber geometry.

The combination of preform downfeed speed and capstan drawing speeddetermine the diameter of fiber produced by the drawing process for agiven drawing temperature. A diameter monitoring system can beconfigured in a feedback loop to enable control of, e.g., the capstanspeed, by the diameter monitors based on a diameter setpoint and controlalgorithm. For the drawing furnace zone temperatures recited above fordrawing a PES-As₂Se₃—Sn preform of 20 cm in diameter and 30 mm inlength, a downfeed speed of between about 0.002 mm/sec-0.004 mm/sec anda capstan speed of between about 0.7 m/sec-3 m/sec produces a fiber of adiameter between about 1200 μm and 500 μm and a length of severalhundred meters. As can be recognized, a reduction in draw speedincreases the resulting fiber diameter. Within the fiber, layers of thepreform are reduced in thickness by a factor of ˜20-100. In accordancewith the invention, a preform can be drawn multiple times to reduce thefinal resulting fiber geometry correspondingly.

The drawdown ratio between a fiber preform and the resulting fiber isnot precise; specifically, the preform layer thickness drawdown ratiodoes not always correspond precisely to the fiber's outer diameterdrawdown ratio. This can be due to a number of factors, including, e.g.,reduction of hollow core or other hollow spaces within the preform. Therelationship between the layer and outer diameter drawdown ratios isfound to be closer to 1:1 for large-diameter, low-tension drawprocedures. High-temperature, low-tension draw procedures can tend toproduce fibers having layers thicker than predicted by the outerdiameter reduction ratio, due, e.g., to partial collapse of hollowregions. It is found, however, that such effects are fairly reproducibleand can be predicted based on experimental history.

Upon completion of the fiber drawing operation, there is produced afiber that can enable optical transmission, separate and independentelectrical transmission, and optoelectronic device operation. Theconducting and semiconducting fiber elements therefore are provided tobe functional in at least one aspect of transmission or device operationand the insulating fiber elements can be provided for electrical and/oroptical isolation as well as for functionality in at least one aspect oftransmission or device operation.

It is to be recognized that while it can be preferred to employconducting, semiconducting and insulating preform materials, the fiberthat results from the draw process can exhibit altered materialconductivities given the scale of feature sizes and cross-sectionalelement dimensions of the drawn fiber. For example, the conditions ofthe fiber drawing and/or the structural and dimensional changes thatresult from the drawing could render a semiconducting or metal preformmaterial insulating, or an insulating preform material conducting.Further, the energy band structure of materials provided in a preformcan be altered by the fiber drawing and/or resulting dimensionalchanges, and can change their conductivity correspondingly, given thescale of fiber feature sizes. In addition, it is recognized that one ormore constituents can be incorporated into preform materials that adjustthe materials' conductivity upon fiber drawing. For example, conductingfilaments, such as carbon fibers, can be included in a preform materialsuch as polymer whereupon drawing, the spacing between the fibers isreduced, leading to a change in polymer conductivity.

The invention contemplates employing these and other phenomena toproduce a drawn fiber of conducting, semiconducting and insulatingmaterials from a preform of materials that may not be conducting,semiconducting and insulating. A corresponding process flow 11 isdescribed in the flow chart of FIG. 1B. In a first process step 13, afiber preform is assembled to include a plurality of distinct materials.Then the assembled fiber preform is consolidated 15 if necessary forproducing intimate interfaces as explained previously. Finally, thefiber preform is drawn 17 into a fiber of conducting, semiconducting,and insulating materials. In accordance with the invention, it is theconducting, semiconducting and insulating material properties of thedrawn fiber that are to be achieved; and it is recognized that thepreform need not provide a one-to-one correspondence in materialconductivity with the fiber while still enabling this desired result.Shifts in the energy band structure of preform materials, dimensional,structural, and other changes in preform material constituents, andother such phenomena impacted by the fiber draw process can be employedto produce a conducting, semiconducting and insulating fiber geometry inaccordance with the invention.

EXAMPLES

Referring to FIG. 4, a fiber 90 for conducting both photons andelectrons was produced in accordance with the invention. A photonconducting region was provided as a hollow fiber core 92 around whichwas provided a multilayer photonic bandgap structure 94 of alternatingsemiconducting and insulating material layers. The bandgap structureexhibited a photonic bandgap at the wavelength corresponding to photontransmission. Sixty cylindrical strands 96 each having an Sn core andpolymer cladding were provided around the bandgap structure, andadditional polymer reinforcement material 98 was provided around thestrands.

While this example bandgap structure is an omnidirectional reflectingmirror, fibers of the invention are not limited to such; the bandgapstructure need not be a multilayer or 1D photonic bandgap structure andinstead can exhibit a 2D or 3D photonic bandgap employing structureshaving periodicities in more than one direction. In the bandgapstructure shown, the wavelengths at which photons are transmitted arecontrolled by the period length of the dielectric mirror of thestructure. A change in the period length thereby changes thetransmission wavelength.

The fiber preform for this geometry was assembled by wrapping a PEIfilm, coated on both sides with a layer of As₂Se₃, around a pyrex tube.Specifically, a 2.6 μm-thick layer of As₂Se₃ was evaporated onto bothsides of a PEI film of 8 μm in thickness. The semiconducting materialwas chemically synthesized in the manner described previously. Highpurity As and Se elements were placed into a quartz tube under anitrogen atmosphere. The tube was heated to 330° C. for one hour at arate of 1° C./min under vacuum to remove surface oxide, As₂O₃, and thencooled to room temperature at 1° C./min. The tube was then sealed undervacuum of about 10⁻⁵ Torr. The resulting ampoule was heated to 700° C.at a rate of 2° C./min in a rocking furnace, held vertical for 10 hours,and then rocked for 12 hours to increase mixing and homogenization. Theliquid was then cooled to 550° C., and quenched in air and water. It wasthen annealed for one half hour to about 180° C. and then graduallycooled to room temperature. The synthesized chalcogenide semiconductorwas thermally evaporated onto both sides of the PEI film, the twosemiconducting layers being ¼ the polymer film thickness.

The coated polymer film was then rolled around a pyrex tube having anouter diameter of 16 mm. The diameter of the tube was selected inconcert with the polymeric insulator and semiconductor layerthicknesses, the required fiber inner core diameter, and the desiredbandgap wavelength. A PEI layer was provided as the outermost layer ofthe material pair. Eight pairs of As₂Se₃/PEI layers were wrapped aroundthe tube. Polymer-coated Sn strands were then produced in the mannerdescribed above. 5 mm diameter Sn wires were each wrapped with a layerof 7.5 mm-thick PEI film. The ends of the wrapped wires were coated witha polymer solution of 20% PES, 80% N,N-Dimethylacetamide.

Each of the polymer-coated metal strands was then attached to the PEIlayer by applying a polymer solution of 20% PES, 80%N,N-dimethylacetamide on the PEI film and sticking the strands to thefilm. Additional layers of PEI film were then wrapped around the metalstrands. The resulting preform was then consolidated at a temperature of260° C. and a pressure of 10⁻³ Torr. After consolidation, the preformwas immersed in a liquid HF bath or 3 hours to selectively etch away thepyrex tube.

With the sacrificial pyrex tube removed, the finalized preform was thendrawn under conditions with a top zone temperature of 192° C., a drawtemperature of 302° C., a downfeed speed of 0.003 mm/min and a capstanspeed of 1 m/min. This resulted in the preform being drawn down to afiber including an As₂Se₃ layer thickness of 150 nm, a PEI layerthickness of 280 nm, and a Sn metal wire diameter of about 8 μm.

FIG. 5 is a plot of experimentally measured optical transmission spectraof the dual electron-photon conducting fiber configuration of FIG. 4 forthree different fiber outer diameters, namely, 980 μm, 1030 μm, and 1090μm. Optical transmission measurements were performed using a BrukerTensor 37 FTIR spectrometer with an InGaAs detector, in the nearinfrared region. Both ends of the fiber were coated with a thick layerof gold to ensure light coupling to the hollow core only, as well as tofacilitate electrical conduction experiments. The fibers exhibitedtransmission peaks corresponding to the fundamental and second-orderphotonic bandgaps, which for the 980 μm-diameter fiber were located at1.62 μm and 0.8 μm, respectively. As explained previously, the positionsof the photonic bandgaps are determined by the lattice period of thelayered mirror structure, which is in turn controlled by the final fiberdiameter.

FIG. 6 is a plot of electrical current as a function of voltage appliedacross the metal strands of the fiber of FIG. 4. The electrical currentof the strands was measured directly under the applied voltage. Theelectron transport properties of the fiber were found to be ohmic overthe range of measurement. It was also found that electrical contact tothe fiber could be achieved using conventional solder due to the highglass transition temperature of the PEI and PES polymers employed in thefiber structure.

The photonic band structure and theoretical optical transmission of thefiber of FIG. 4 was predicted using a general expression for the radialoutgoing flux from the cylindrical fibers. The plot of FIG. 7Arepresents the resulting band diagram. Darker shaded areas correspond tobandgap regions having decaying solutions for outgoing flux. Lightershaded areas outside the bandgaps correspond to regions where lightcouples to radiating modes that are transmitted through the mirrorstructure and are therefore not localized for transmission within thehollow fiber core.

The optical transmission properties of these high over-moded fibers isset by the small intermodal separation, which is inversely proportionalto the square of the fiber radius. Thus, a fiber core radius of 250λ, or400 μm, is expected to have ˜10⁵ modes, making it difficult to observethe individual dispersion curves of the propagating modes in a fullscale band structure. The inset image of the plot is a magnified segmentof the guided modes near the light line, where the dispersion curves ofthe first three propagating fiber modes with angular momentum of 1 canbe observed by the light colored stripes that indicate local minima inthe outgoing flux, and therefore a strong confinement of the fieldwithin the hollow core.

FIG. 7B is a plot of the calculated transmission spectrum for the fiber,based on the leaky mode technique. This calculated spectrum compareswell with the measured spectra of the plot of FIG. 5, demonstrating hightransmission at wavelengths corresponding to the first and second orderphotonic bandgaps.

Referring now to FIG. 8, a fiber was produced in accordance with theinvention configured with a 500 μm diameter semiconducting core region102 of As₂Se₃ chalcogenide glass directly contacted by two Sn metalelectrodes 104 of 65 μm in radial thickness and 180 μm incircumferential length, along the longitudinal length of the fiber. Alayer of PES was provided around the metal electrodes.

The fiber preform was produced in the manner described previously, withthe As₂Se₃ chalcogenide glass core chemically synthesized. The metalelectrodes were formed of Sn foil cut into rectangles and inserted intoregions cut into a PES film. The Sn-PES structure was then wrappedaround the glass core, and additional PES layers were wrapped around theassembly.

The preform was consolidated at a pressure of 10⁻³ Torr and atemperature of 260° C. The preform was then drawn under conditions witha top zone temperature of between 190° C.-230° C. and a draw zonetemperature of between 290° C. and 295° C. A downfeed speed of 0.003mm/min and a capstan speed of 1 m/min were employed.

Under experimental testing, the electrical resistance of the fiber wasfound to decrease dramatically upon illumination, due to thephoto-induced generation of electron-hole pairs at the surface of thesemiconducting core 102. This demonstration verified the operation offiber as a photodetector device. Once such charges are generated, theyare separated by an electric field produced by a voltage applied betweenthe metal electrodes and are swept towards opposing electrodes.

FIG. 9A is a plot of the photo-response of the photodetector fiber. Thisphoto-response was measured using a Yokagawa pico-ampere meter and aHewlett Packard 4140B DC voltage source. The fiber was illuminated usingwhite light from a quartz-tungsten-halogen lamp, which was measured toproduce a conductivity enhancement of up to two orders of magnitude. Thelinear behavior of the I-V plot of FIG. 9A is indicative of ohmicbehavior in the structure; no Schottky effects at themetal-semiconductor junction were observed over the range ofmeasurement. It is to be noted that the photodetector fiber couldalternatively be operated in a capacitive rather than resistivedetection mode.

FIG. 9B is a plot the dependence of photodetector fiber's conductivityon optical illumination intensity, which was found to be linear over therange of measurement. The electric field lines produced by the appliedvoltage were simulated using finite element techniques and are shown inthe inset of FIG. 9A.

The photodetector fiber response is reminiscent of ametal-semiconductor-metal photodetector (DMSM), and provides particularadvantages over that structure. In particular, the invention enablesadaptation of the photodetector fiber for a range of applications. Forexample, the hybrid fiber array of FIG. 3G can be configured as an arrayof photodetector fibers. Other configurations can also be arranged toenhance and/or exploit the photoresponse of the photodetector fiber.

The photosensitivity of the photodetector fiber was found to scaleinversely with its diameter, and thus fibers with smaller elementdiameters are understood to exhibit increased photosensitivity. FIG. 10Ais a plot of photodetector fiber resistance, R_(I), as a function offiber diameter. It can be shown from simple scaling arguments that areduction in fiber dimensions will not change the dark resistance,R_(d). However, upon illumination, a conducting layer is formed alongthe circumference of the fiber having a thickness which is determinedsolely by the penetration depth of the illuminated light, which isindependent of diameter. The resistance of this layer scales linearlywith its length, which in turn is proportional to the fiber diameter,resulting in the observed linear dependence of the illuminated fiber'sresistance diameter.

It was found that the photodetector fiber's photogenerated current alsodepends on the length of the illuminated portion of the fiber. FIG. 10Bis a plot of resistance as a function of illuminated fiber length. Thisresult demonstrates that the photodetector fiber exhibits trulydistributed characteristics. The photodetection functionality iscontinuous along the photodetector fiber length, in contrast toconventional photodetecting elements that typically are point-likeobjects limited the micron scale. The electronic mobility edge of As₂Se₃glass corresponds to a wavelength of 650 nm, making this fiberconfiguration an efficient photo-detector over the visible range as wellas near IR range. This detection range could be readily expanded throughselective compositional changes to the core glass.

The ability to combine precise dimensional and geometrical control,achieve small feature sizes and maintain axial uniformity along extendedlengths of fiber while at the same time facilitating intimate contactand adhesion between conducting, semiconducting and insulating materialspaves the way for a broad range of functional fibers for electronic andphotonic applications. It is recognized, of course, that those skilledin the art may make various modifications and additions to theembodiments described above without departing from the spirit and scopeof the present contribution to the art. Accordingly, it is to beunderstood that the protection sought to be afforded hereby should bedeemed to extend to the subject matter claims and all equivalentsthereof fairly within the scope of the invention.

1. A method for producing a fiber comprising: assembling a fiber preform including conducting, insulating, and semiconducting materials; and drawing the preform into a fiber.
 2. A method for producing a fiber comprising: assembling a fiber preform including a plurality of distinct materials, and drawing the preform into a fiber comprising conducting, insulating, and semiconducting materials.
 3. The method of either of claims 1 or 2 further comprising consolidating the fiber preform prior to drawing the preform into a fiber.
 4. The method of claim 3 wherein the fiber preform consolidating comprises heating the fiber preform under vacuum.
 5. The method of either of claims 1 or 2 wherein the fiber drawing is at a draw temperature at which viscosity of the materials is less than about 10⁸ Poise.
 6. The method of either of claims 1 or 2 wherein the fiber drawing is at a draw temperature greater than a characteristic melting temperature of the conducting material.
 7. The method of either of claims 1 or 2 wherein the fiber drawing is at draw temperature between about 280° C. and about 315° C.
 8. The method of either of claims 1 or 2 wherein the fiber drawing comprises drawing a fiber having a length greater than about 10 meters.
 9. The method of claim 8 wherein the fiber drawing comprises drawing a fiber having a length greater than about 20 meters.
 10. The method of claim 9 wherein the fiber drawing comprises drawing a fiber having a length greater than about 50 meters.
 11. The method of claim 10 wherein the fiber drawing comprises drawing a fiber having a length greater than about 100 meters.
 12. The method of either of claims 1 or 2 wherein fiber preform assembly comprises assembling a preform having a length less than about 100 cm.
 13. The method of claim 12 wherein fiber preform assembly comprises assembling a preform having a length less than about 50 cm.
 14. The method of either of claims 1 or 2 wherein the fiber drawing comprises drawing a fiber having a length that is at least about 100 times greater than a characteristic length of the fiber preform.
 15. The method of either of claims 1 or 2 wherein the fiber drawing comprises drawing a fiber having a diameter that is at least about 10 times less than a characteristic diameter of the fiber preform.
 16. The method of either of claims 1 or 2 further comprising: cutting the fiber into a plurality of sub-fibers; assembling the sub-fibers into a hybrid fiber preform; and drawing the hybrid fiber preform into a hybrid fiber.
 17. The method of claim 16 wherein assembling a hybrid fiber preform comprises including sub-fibers having dissimilar cross-sections in the preform.
 18. The method of either of claims 1 or 2 wherein the insulating material comprises a polymeric insulating material.
 19. The method of claim 18 wherein the polymeric insulating material comprises a high glass-transition-temperature material.
 20. The method of claim 18 wherein the polymeric insulating material comprises poly-ether imide.
 21. The method of claim 18 wherein the polymeric insulating material comprises poly-ether sulfone.
 22. The method of claim 18 wherein the polymeric insulating material comprises poly-methyl methacrylate.
 23. The method of claim 18 wherein the polymeric insulating material comprises a polymeric rod.
 24. The method of claim 18 wherein the polymeric insulating material comprises a polymeric tube.
 25. The method of claim 18 wherein the polymeric insulating material comprises a polymeric layer.
 26. The method of either of claims 1 or 2 wherein the insulating material comprises an optical transmission element.
 27. The method of either of claims 1 or 2 wherein the insulating material comprises an optoelectronic device element adjacent to and in contact with a conducting element.
 28. The method of either of claims 1 or 2 wherein the conducting material comprises a metal.
 29. The method of claim 1 further comprising applying an oxidation inhibitor to the conducting material before drawing the preform into a fiber.
 30. The method of claim 1 further comprising applying a wetting promoter to the conducting material before drawing the preform into a fiber.
 31. The method of claim 1 further comprising applying a flux to the conducting material before drawing the preform into a fiber.
 32. The method of either of claims 1 or 2 wherein the conducting material comprises a metal.
 33. The method of either of claims 1 or 2 wherein the conducting material comprises a metal alloy.
 34. The method of claim 32 wherein the metal comprises Sn.
 35. The method of claim 32 wherein the metal comprises at least one metal strand.
 36. The method of claim 32 wherein the metal comprises a metal foil.
 37. The method of claim 32 wherein the metal comprises a metal layer.
 38. The method of either of claims 1 or 2 wherein the conducting material comprises an electrical transmission element.
 39. The method of either of claims 1 or 2 wherein the conducting material comprises an electrode element.
 40. The method of either of claims 1 or 2 wherein the conducting material comprises an optoelectronic device element adjacent to and in contact with a semiconducting element.
 41. The method of either of claims 1 or 2 wherein the conducting material comprises an electrical conducting element that is geometrically confined by at least one of the semiconducting and insulating materials.
 42. The method of either of claims 1 or 2 wherein the semiconducting material comprises a semiconducting chalcogenide glass.
 43. The method of claim 42 wherein the semiconducting material comprises (As₄₀Se₆₀)_(1-x)Sn_(x).
 44. The method of claim 42 wherein the chalcogenide glass comprises As₂Se₃.
 45. The method of either of claims 1 or 2 wherein the semiconducting material comprises an optical transmission element.
 46. The method of either of claims 1 or 2 wherein the semiconducting material comprises an optoelectronic device element adjacent to and in contact with an electrically conducting element.
 47. The method of either of claims 1 or 2 wherein the semiconducting material comprises a semiconducting rod.
 48. The method of either of claims 1 or 2 wherein the semiconducting material comprises a semiconducting layer.
 49. The method of either of claims 1 or 2 wherein preform assembly comprises wrapping layers of the materials around a rod.
 50. The method of either of claims 1 or 2 wherein preform assembly comprises wrapping layers of the materials around a tube.
 51. The method of either of claims 1 or 2 wherein preform assembly comprises evaporating semiconducting material on a layer of insulating material.
 52. The method of either of claims 1 or 2 wherein preform assembly comprises wrapping a layer of insulating material that is coated with a layer of semiconducting material around a preform element.
 53. The method of either of claims 1 or 2 wherein preform assembly comprises wrapping a metallic foil layer around a preform element.
 54. The method of claim 53 further comprising applying at least one of an oxidation inhibitor and a wetting promoter to at least one of the foil and a material adjacent to the foil.
 55. The method of either of claims 1 or 2 wherein preform assembly comprises wrapping a polymer film around a preform element.
 56. The method of either of claims 1 or 2 wherein preform assembly comprises positioning at least one conducting strand adjacent to a preform element.
 57. The method of claim 56 further comprising applying at least one of an oxidation inhibitor and a wetting promoter to at least one of a conducting strand and a material adjacent to the conducting strand.
 58. The method of claim 56 further comprising wrapping a polymer film around the at least one conducting strand and preform element.
 59. The method of claim 56 wherein positioning at least one conducting strand comprises applying a liquid polymer solution to at least a portion of a preform element and arranging the strand on the polymer solution-coated portion of the preform element.
 60. The method of either of claims 1 or 2 wherein preform assembly comprises positioning at least one conducting electrode in a polymer film and wrapping the polymer film around a preform element.
 61. The method of claim 60 further comprising applying at least one of an oxidation inhibitor and a wetting promoter to at least one of the conducting electrode and a material adjacent to the electrode.
 62. The method of claim 60 wherein the preform element comprises a semiconducting rod.
 63. The method of either of claims 1 or 2 wherein preform assembly comprises positioning at least one conducting electrode in a polymer tube sidewall and positioning the polymer tube around a preform element.
 64. The method of claim 63 further comprising applying at least one of an oxidation inhibitor and a wetting promoter to at least one of the conducting electrode and a material adjacent to the electrode.
 65. The method of claim 63 wherein the preform element comprises a semiconducting rod.
 66. The method of either of claims 1 or 2 wherein preform assembly comprises forming at least one hole in a preform element and positioning at least one of the materials in the hole.
 67. The method of either of claims 1 or 2 wherein preform assembly comprises arranging a sacrificial preform element with the materials to define a hollow fiber region, and removing the sacrificial preform element prior to fiber drawing.
 68. The method of claim 67 wherein the sacrificial preform element comprises a rod.
 69. The method of claim 67 wherein the sacrificial preform element comprises a tube.
 70. The method of claim 67 further comprising thermally consolidating the preform under vacuum prior to removing the sacrificial preform element from the preform.
 71. The method of claim 67 wherein sacrificial preform element removal comprises selective chemical etching of the sacrificial preform element. 